It is well known that certain treatments of petroleum, e.g., hydrogenation desulfurization, require the use of certain types of catalysts. Typical catalysts for hydrogenation desulfurization of the heavy crudes are based on molybdenum oxide combined with oxides of cobalt or nickel, all sulfidized on a porous alumina support material. The particular catalysts are deposited on the carrier and are used to remove sulfur, oxygen and nitrogen from petroleum. During the desulfurization process, heavy metals such as vanadium and nickel, as well as carbon, sulfur and other contaminants originating in the heavy oil, deposit on the catalyst until the catalyst wears out and loses activity. Such a worn out catalyst, commonly known as a spent catalyst, must be replaced with a new charge of a fresh catalyst to continue the reaction.
In the past, spent catalysts were usually deposited in landfills, but this method has become increasingly expensive and undesirable because of environmental restrictions and liabilities. In addition, there is a considerable amount of valuable heavy metals which are being discarded.
Several processes have been proposed which relate to the recovery of such heavy metals. These methods basically use the extraction of the heavy metals with an aqueous acid or an alkali solution. Usually, the spent catalyst is calcined and oxidized at a relatively high temperature of 500.degree. C. or higher before extraction with an acid or alkali. This calcination process converts all of the heavy metals to the oxide forms which are readily decomposable and extractable by acid or alkali. Other processes comprise leaching to remove the catalytic material by chemically combining it with another material in the gaseous or liquid phase. Still other processes involve melting the catalyst material and mechanically separating the alumina containing melt component from the metal alloy residue.
None of these processes is entirely satisfactory, primarily due to the high volume of catalyst support needed for processing relative to the low volume of valuable metals recovered. In many cases, the economics of the recovery process do not justify recovering the valuable metals from the spent catalyst even though the total amount of valuable metals is significant. There still exists a need in the industry for an efficient process that economically recovers substantially all of the valuable metals from spent catalysts and eliminates the environmental problems caused by the disposal of spent catalysts.